1. Field of the Invention
The present invention relates generally to methods of and apparatuses for detecting a malfunction of a displacement detector of the type arranged to prevent its two output voltages from coinciding with each other.
2. Description of the Related Art
In Japanese Patent Laid-open Publication No. HEI-7-332910, the assignee of the present patent application proposed a displacement detector capable of detecting displacement of a given object with high accuracy and sensitivity. The proposed displacement detector will be outlined below with reference to FIGS. 4A, 4B and 4C.
As shown in FIG. 4A, the proposed displacement detector 1 includes a core 3 movable (in arrowed directions X1 and X2) within a detecting coil 2 in response to displacement of a predetermined object of detection. The detecting coil 2 is connected at one end 2a to one end of a pulse power supply 5 and connected at the other end to the other end of the pulse power supply 5 via a reference resistance Rf so as to detect a transient response voltage VO across the reference resistance Rf.
Where an inductance of the detecting coil 2 when the core 3 is in the neutral or center position within the detecting coil 2 is set to a value L and the reference resistance Rf is set to be sufficiently greater than an internal resistance of the detecting coil 2, the internal resistance of the detecting coil 2 can be ignored, and there can be obtained an equivalent circuit of the detector 1 as shown in FIG. 4B. If the inductance L of the detecting coil 2 and the reference resistance Rf, connected in series with each other, are driven by a rising-amplitude portion of the supplied voltage (with a peak value VI) whose half period (T/2) is sufficiently greater than a time constant .tau. determined by the inductance L and reference resistance Rf (.tau.=L/Rf), a transient response waveform results as shown in FIG. 4C and a transient response voltage VO after lapse of time tk may be expressed by the following equation: EQU VO=VI*e.sup.- (Rf/L)*tk Equation (1)
If the inductance L of the detecting coil 2 and the reference resistance Rf are driven by a decaying-amplitude portion of the supplied voltage in a condition where the core 3 has moved in the direction X1 by a given amount with the inductance L of the detecting coil 2 decreased by an amount .DELTA.L (namely, L-.DELTA.L), a transient response voltage VO1 after lapse of time tk may be expressed by the following equation: EQU VO1=VI*e.sup.- (Rf/L-.DELTA.L)*tk Equation (2)
The inductance variation amount .DELTA.L, corresponding to the displacement of the core 3 in the X1 direction, may be expressed by Equation (3) below after calculating the inductance L from the transient response voltage VO (Equation (1)) when the core 3 is in the center position within the detecting coil 2 (presenting no displacement) and calculating the inductance (L-.DELTA.L) from the transient response voltage VO1 (Equation (2)) when the core 3 has displaced in the X1 direction by the given amount: EQU .DELTA.L=Rf*tk{ln.sup.-1 (VO1/VI)-ln.sup.-1 (VO/VI)} Equation (3)
Assuming that the time tk is fixed, the inductance variation amount .DELTA.L for the displacement of the core 3 in the X1 direction will correspond to a ratio between the transient response voltage VO or VO1 and the peak value VI of the supplied power from the supply 5. Thus, the inductance variation amount .DELTA.L can be determined independently of the peak value VI and frequency f (1/T) of the supplied power from the supply 5, and the displacement amount in the X1 direction can be determined from the thus-determined inductance variation amount .DELTA.L. The displacement amount in the X1 direction can also be determined by detecting the transient response voltages VO and VO1 because the inductance variation amount .DELTA.L corresponds to the voltages VO and VO1.
Whereas determination of the displacement amount in the X1 direction has been described above in relation to the case where the inductance L of the detecting coil 2 and the reference resistance Rf are driven by the decaying-amplitude portion of the supplied voltage from the supply 5, the displacement amount in the X1 direction can also be determined by detecting transient response voltages corresponding to the voltages VO and VO1 in a situation where the inductance L of the detecting coil 2 and the reference resistance Rf are driven by the rising-amplitude portion of the supplied voltage.
FIGS. 5A and 5B show a displacement detector provided with two detecting coils and an equivalent circuit of the displacement detector. In FIG. 5A, the displacement detector 10 includes a displacement sensor 11, a pulse power supply 15 and two reference resistances Rf. The displacement sensor 11 includes a core 13 displaceable in opposite directions (upward and downward in the figure), and first and second detecting coils 12A and 12B disposed symmetrically along the displacement directions of the core 13 so that their inductances vary in a differential manner. The first detecting coil 12A has one end 12a connected to one end of one of the reference resistances Rf and another end 12b connected with one end 12c of the second detecting coil 12B. Further, the second detecting coil 12B is connected at the other end 12d to one end of the other reference resistance Rf, and the other ends of the two reference resistances Rf are connected with each other and grounded. The pulse power supply 15 (with a peak value VI) is coupled to junctions between the ends 12b and 12c of the first and second detecting coils 12A and 12B and between the two reference resistances Rf, so that a voltage generated across one of the resistances Rf is taken out at a first detecting terminal S1 while a voltage generated across the other resistances Rf is taken out at a second detecting terminal S2.
As shown in FIG. 5B, the equivalent circuit of the displacement detector 10 of FIG. 5A comprises a bridge circuit 14 that includes inductances L1 and L2 of the above-mentioned detecting coils 12A and 12B and two reference resistances Rf. Power from the pulse power supply 15 is supplied to the bridge circuit 14 so as to take out transient response voltages VS1 and VS2 at detecting terminals S1 and S2. Potential difference VD between the detecting terminals S1 and S2 (hereinafter called a "detected voltage") equals a difference between the two transient response voltages VS1 and VS2 (VS1-VS2).
FIGS. 6A and 6B are waveform diagrams of the transient response voltages produced in the displacement detector of FIG. 5A; FIG. 6A shows a transient response voltage waveform where the rising and decaying wave segments of the supplied pulse power are equal to each other in time length, while FIG. 6B shows a transient response voltage waveform where the rising and decaying wave segments of the supplied pulse power are different from each other in time length. In the example of FIG. 6A, the time length (T/2) of each decaying wave segment is set to be sufficiently longer than a time constant of an integrator circuit, composed of the inductances L1 and L2 of the detecting coils and the reference resistances Rf, so that the transient response voltages reach zero volt at time T/2. In contrast, the time length (T1) of each decaying wave segment in FIG. 6B is set to be shorter so that the transient response voltages do not reach zero volt at time T1. In each of the examples of FIGS. 6A and 6B, however, the time length of each rising wave segment is set to be sufficiently longer than the time constant of the integrator circuit so that the transient response voltages reach the peak value V1 at time T or T2.
Consider, for example, a case where the core 13 of the displacement sensor 11 in the displacement detector 10 of FIG. 5A is displaced by a given amount in the X1 direction from the center position (exactly between the two detecting coils 12A and 12B), in accordance with which the inductance L (i.e., inductance at the center position) of the first detecting coil 12A has decreased to L1 while the inductance L of the second detecting coil 12B has increased to L2. In this case, the time constant of the transient response voltage VS1 taken out at the detecting terminal S1 (L1/Rf) becomes smaller than that of the transient response voltage VS2 at the detecting terminal S2 (L2/Rf), due to the relationship of L1&lt;L2. Thus, the rise and fall time of the transient response voltage VS1 will be shorter than those of the transient response voltage VS2, as shown in FIGS. 6A and 6B.
When the core 13 is displaced in the X1 direction, the detected voltage VD between the detecting terminals S1 and S2 (=VS1-VS2) assumes a negative (minus) polarity during each decaying time of the supplied pulse power but assumes a positive (plus) polarity during each rising time Conversely, when the core 13 is displaced in the X2 direction, the detected voltage VD between the detecting terminals S1 and S2 assumes a positive polarity during each decaying time of the supplied pulse power but assumes a negative polarity during each rising time. In this way, the displacement amount in the X1 or X2 direction can be determined from the absolute value of the detected voltage VD, and the displacement direction can be determined from the polarity of the detected voltage VD.
Where the displacement detector is arranged in such a manner that the detected voltage VD is determined during the decaying time of the supplied pulse power, the maximum absolute value of the detected voltage VD (negative polarity) can be detected at a time point tM in the example of FIG. 6A and at a time point T1 in the example of FIG. 6B. Note that the same maximum absolute value of the detected voltage VD (negative polarity) can also be detected in the example of FIG. 6B by setting the time T1 to coincide with the time tM. Thus, even with the same maximum displaceable amount of the displacement sensor 11, a highly sensitive displacement detector 10 is achieved, by just setting detection timing and fall time of the supplied pulse power such that the maximum value of the detected voltage VD can be determined.
FIG. 7 is a block diagram of a torque detector employing the displacement detector as shown in FIG. 5A, and FIGS. 8A, 8B, 8C, 8D, 9A and 9B are waveform diagrams explanatory of behavior of the torque detector shown in FIG. 7. As shown in FIG. 7, the torque detector 40 generally comprises a torque sensor body 41 using the displacement detector of FIG. 5A, and a torque detecting unit 46. The torque sensor body 41 includes an input shaft 42, an output shaft 43, a torsion bar (not shown) interconnecting the input and output shafts 42 and 43, an axially-movable core 44, first and second detecting coils 45A and 45B, and two reference resistances (not shown) that are similar to the reference resistances Rf of FIG. 5A.
As torque is applied to the input and output shafts 42 and 43, a torsional angle proportional to the applied torque is produced in the torsion bar. This torsional angle is converted into an axial displacement of the core 44, by cooperation between a pin (not shown) coupled to both the shafts 42 and 43 and spiral and vertical grooves (not shown) formed in the core 44. The axial displacement of the core 44 is detected as inductance variations (.DELTA.LT) of the first and second detecting coils 45A and 45B, and the inductance variations (.DELTA.LT) are determined from transient response voltages VS1 and VS2 that result from a pulse voltage VI supplied to a bridge circuit composed of the detecting coils 45A and 45B and the two reference resistances.
The torque detecting unit 46 includes a pulse generator circuit 51 for supplying the pulse voltage VI to the torque sensor body 41, low-pass filters 47A and 47B for eliminating high-frequency switching noise from the transient response voltages VS1 and VS2, output from the sensor body 41, to thereby provide noise-eliminated transient response voltages Va1 and Va2, and bottom holding transient response voltages Va1 and Va2, and bottom holding circuits (48A, 458B) for temporarily holding respective bottom voltages VT1 and VT2 (e.g., voltage values at the time point T1 of FIG. 6B) of the transient response voltages Va1 and Va2. The torque detecting unit 46 further includes a differential amplifier 49 for calculating a difference between the bottom voltages VT2 and VT1 (VT2-VT1) and amplifying the calculated difference by a gain G1 to provide a difference voltage Vb, and an inverter amplifier 50 for inverting the difference voltage Vb and shifting the inverted difference voltage Vb by a reference voltage value (e.g., 2.5 volts) to thereby provide a torque detecting voltage VT.
FIG. 8A shows a waveform of the pulse voltage VI output from the pulse generator circuit 51, and FIG. 8B shows respective waveforms of the transient response voltages VS1 and VS2 output from the bridge circuit of the sensor body 41. The waveforms of the transient response voltages VS1 and VS2 contain switching noise. FIG. 8C shows respective waveforms of the noise-eliminated transient response voltages Va1 and Va2 passed through the low-pass filters 47A and 47B, and FIG. 8D shows waveforms of the bottom voltages VT1 and VT2.
Further, FIG. 9A shows a waveform of the difference voltage Vb obtained by amplifying the difference between the bottom voltages VT2 and VT1 (VT2-VT1) by the gain G1. FIG. 9B shows a waveform of the torque detecting voltage VT obtained by inverting the difference voltage Vb and shifting the inverted difference voltage Vb by the reference voltage value (e.g., 2.5 volts). The torque detecting voltage VT is maintained at the reference voltage of 2.5 volts when no torque is applied, but as torque is applied, it varies linearly in accordance with the direction and magnitude of the applied torque.
The torque detector 40 shown in FIG. 7 is capable of converting the inductance variations of the first and second detecting coils 45A and 45B into voltage variations to thereby provide two output voltages (bottom voltages) VT1 and VT2, and it is arranged to prevent the output voltages (i.e., bottom voltages) VT1 and VT2 from coinciding with each other over a predetermined operating range (i.e., torque detecting range) of the torque detector 40 (see FIG. 9A).
Examples of the conventional control devices, intended for performing various control on the basis of torque detected by such a torque detector arranged to prevent the output voltages (bottom voltages) VT1 and VT2 from coinciding with each other as mentioned, include the electric power steering apparatus for detecting manual steering torque via the torque detector to control a steering assist from an electric motor in accordance with the detected manual steering torque. When the output voltages (bottom voltages) VT1 and VT2 get out of a predetermined voltage range, such conventional control devices determine that the torque detector is malfunctioning and stop their control operation. Also, when the sum or average of the output voltages (bottom voltages) VT1 and VT2 departs from or gets out of a predetermined voltage range, the control devices determine that the torque detector is malfunctioning and stop their control operation.
For example, with the torque detector where the allowable voltage range of the output voltages (bottom voltages) VT1 and VT2 in normal conditions is 0.2-4.8 volts and which is arranged in such a manner that the average of the voltages VT1 and VT2 is generally maintained at 2.5 volts, the conventional control devices determines the torque detector as malfunctioning when any one of the following conditions is detected:
(1) one of the output voltages (bottom voltages) VT1 and VT2 is not greater than 0.2 volts;
(2) one of the output voltages VT1 and VT2 is not smaller than 4.8 volts;
(3) the average of the output voltages VT1 and VT2, i.e., (VT1+VT2)/2, is not greater than 2 volts; and
(4) the average of the output voltages VT1 and VT2 is not smaller than 3 volts.
However, the conventional control devices, which are arranged to detect a malfunction of the torque detector on the basis of the preset allowable voltage ranges of the individual output voltages VT1 and VT2, their sum and their average, can not detect when the output voltages (bottom voltages) VT1 and VT2 are brought into a "short-circuit" state. For example, the conventional control devices employing the torque detector of FIG. 7 can not detect when the transient response voltages VS1 and VS2 pass into a short-circuit state in a connector CN1 of the sensor body 41, in a connector CN2 of the torque detecting unit 46 or in a cable CA interconnecting the connectors CN1 and CN2, because the output voltages (bottom voltages) VT1 and VT2 from the detecting unit 46 would both amount to about 2.5 volts (VT1=VT2=2.5 volts).